Lightening unit

ABSTRACT

A backlighting unit for planar and homogeneous illumination has a cold-cathode fluorescent lamp that is mounted one narrow side of a waveguide plate. The light emitted from the backlighting unit is emitted from one of the two large surfaces of the waveguide plate. The light is deflected from one narrow side to one large surface by prisms which are also mounted on one large surface. To homogenize the emitted light, the prisms are arranged in irregular fashion. Homogenization is advantageous in particular if an L-shaped or U-shaped lamp, which illuminates several narrow sides of the waveguide plate, is used as the lamp.

FIELD OF THE INVENTION

The present invention relates to a backlighting unit.

BACKGROUND INFORMATION

A conventional backlighting unit is described in U.S. Pat. No.5,390,276. The principal element of this backlighting unit is a lightguide, made of transparent material (for example Plexiglas), whichpossesses roughly the shape of a parallelepipedal plate. One of the endsurfaces is used as a light entry surface, and the other end surfacesare treated, for example by metal-coating, in such a way that no lightcan emerge through them from the light guide. One of the two largecovering surfaces is configured as a light exit surface, and smallprisms are applied on the other large covering surface. The prisms allhave the same cross-sectional shape and cross-sectional area, and allextend parallel to the light entry surface. The lamp is located in theimmediate vicinity of the light entry surface. A cold-cathodefluorescent lamp, in the form of an elongated tube, is used as the lamp.The lamp is arranged in front of the light entry opening in such a waythat as large a percentage as possible of the light radiation generatedby the lamp is coupled into the waveguide. A reflector, which alsoconveys to the waveguide the light that is not irradiated directly intothe waveguide, is additionally provided on the side of the lamp facingaway from the waveguide.

As a result of the prisms arranged on the underside of the waveguide,the light present in the waveguide is coupled out in particularlyefficient fashion by the fact that the light is reflected from theprisms applied on the one covering surface to the opposing coveringsurface, from which it can then emerge. It is important to consider inthe case of this arrangement, however, that the coupling-out efficiencyis the same over the entire waveguide. If the rod-shaped lamp is thusreplaced, for example, by an L-shaped or

U-shaped tube, and if several end surfaces are thus used as light entrysurfaces, the result is an inhomogeneous distribution of the lightintensity in the waveguide. The homogeneous coupling-out efficiency thusresults in inhomogeneous backlighting. On the other hand, however, theuse of L-shaped or U-shaped tubes is desirable, since they areparticularly efficient.

SUMMARY OF THE INVENTION

The backlighting unit according to the present invention has, incontrast, the advantage that it can supply high-intensity light which atthe same time possesses an extremely homogeneous intensity distribution.This is advantageous, for example, for installation in liquid crystaldisplays for motor vehicles.

It is particularly advantageous to furnish prisms having the samecross-sectional shape, since the light intensity thus has at all pointsthe same dependency on the angle of view.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a first exemplary embodiment of a backlighting elementaccording to the present invention.

FIG. 1B shows a cross-section of the backlighting unit illustrated inFIG. 1A along AA′ line.

FIG. 1C shows a cross-section of the backlighting unit illustrated inFIG. 1A along BB′ line.

FIG. 2A shows a second exemplary embodiment of the backlighting elementaccording to the present invention.

FIG. 2B shows a cross-section of the backlighting unit illustrated inFIG. 2A along CC′ line.

FIG. 2C shows a cross-section of the backlighting unit illustrated inFIG. 2A along DD′ line.

FIG. 3A shows a third exemplary embodiment of the backlighting elementaccording to the present invention.

FIG. 3B shows a cross-section of the backlighting unit illustrated inFIG. 3A along HH′ line.

FIG. 3C shows a cross-section of the backlighting unit illustrated inFIG. 3A along II′ line.

FIG. 4A shows a second exemplary embodiment of the backlighting elementaccording to the present invention.

FIG. 4B shows a cross-section of the backlighting unit illustrated inFIG. 4A along EE′ line.

FIG. 4C shows a cross-section of the backlighting unit illustrated inFIG. 4A along FF′ line.

FIG. 4D shows a cross-section of the backlighting unit illustrated inFIG. 4A along GG′ line.

FIG. 4E shows a cross-section of the backlighting unit illustrated inFIG. 4A along HH′ line.

FIG. 5 shows a mirror for the backlighting element according to thepresent invention.

DETAILED DESCRIPTION

FIG. 1A shows a first backlighting element. The backlighting element hasa waveguide plate 10 which is configured as an approximatelyparallelepipedal plate made of Plexiglas.

Other materials for the waveguide plate are conceivable, however, andprovision is made therefor, if they have the requisite hightransparency. Waveguide plate 10 is surrounded on three of its narrowsides by a U-shaped light source 12. The light source is in turnenclosed by a reflector 13, also U-shaped. On its underside 16, which isintended to be visible in the depiction selected here, waveguide plate10 has prisms 17. Prisms 17 all have the same cross-sectional shape andthe same cross-sectional dimensions. All the prisms are assumed to belinear (i.e. the ridge lines of the prisms are straight lines). Thearrangement of the prisms is approximately star-shaped, the center pointof the star being located on the center perpendicular through the centerpart of the U-shaped light source 12, far outside the dimensions ofwaveguide 10.

A first cross section through the backlighting unit shown in FIG. 1Aalong section line AA′, which is also shown in FIG. 1A, is depicted inFIG. 1B, identical components having been labeled with identicalreference characters.

FIG. 1C reproduces a further cross section through the backlighting unitdepicted in FIG. 1A, in this case along the second section line BB′.Once again, identical reference characters refer to identicalcomponents.

Light which is emitted from light source 12 moves approximately parallelto light exit surface 14 in waveguide plate 10. When this light strikesthe light exit surface it is totally reflected; when it strikes thenarrow side located opposite, it is reflected back by reflector 13. Butif this light strikes one of prisms 17, it is deflected and thus strikeslight exit surface 14 at an angle which is less than the critical anglefor total reflection. The probability of a light beam striking a prism17 is thus also an indication of the efficiency with which light iscoupled out of waveguide plate 10. In the exemplary embodiment shown inFIGS. 1A-1C, the inhomogeneous light intensity distribution in waveguideplate 10 resulting from the U-shaped configuration of light source 12 iscompensated for by the fact that the coupling-out efficiency is greaterin those parts of the waveguide plate where the intensity of the lightis lower.

It is not essential for the present invention that these prisms belinear. In particular, it is also conceivable, and provision is madetherefor, for the ridge lines of the prisms approximately to follow thegradient of light intensity in the waveguide plate. It is alsoconceivable, and provision is made therefor, for the prisms not to havea triangular cross-sectional shape. In particular, a double-paraboliccross-sectional shape is advantageous, since it allows collimation ofthe coupled-out light. Further possible variants of the backlightingunit according to the present invention result from the fact that theinhomogeneity in the backlighting is not completely compensated for, byway of a coupling-out efficiency which varies over the dimensions of thewaveguide. This is desirable in particular if the backlighting unit isused to backlight a liquid crystal display, and the liquid crystaldisplay is not meant to have a uniform light intensity throughout.

A further exemplary embodiment is shown in FIGS. 2A through 2C. FIG. 2Ashows a plan view of a backlighting unit. The backlighting unit onceagain has an approximately parallelepipedal waveguide plate made of atransparent material, for example Plexiglas. As in FIG. 1A, waveguideplate 10 is surrounded on three of its end faces by a light source 12,behind which is located a reflector 13. Underside 16 of waveguide plate10 visible in FIG. 2A is equipped with linear prisms 17 which are allarranged along the longitudinal axis (i.e. parallel to the two arms ofthe U-shaped light source 12). Prisms 17 are linear prisms, and have thesame cross-sectional shape throughout, both with respect to one anotherand along their longitudinal axis. The cross-sectional area of theprisms varies, however, both among the individual prisms and within oneprism along the longitudinal axis. The cross-sectional area is greatestin the center of that end surface of waveguide plate 10 which is notequipped with a light source, and decreases continuously from there inall directions.

This is also illustrated in the two cross-sectional drawings in FIG. 2Band FIG. 2C.

FIG. 2B shows a section through the backlighting unit depicted in FIG.2A, along section line C′C also depicted in FIG. 2A, identicalcomponents having been given identical reference characters.

FIG. 2C shows a further section through the backlighting unit depictedin FIG. 2A, in this case along section line DD′ which is also shown inFIG. 2A.

For a functional description of the backlighting unit depicted in FIG.2A, reference is made to the description of the backlighting unitdepicted in FIG. 1A. Once again, the light emitted from the backlightingunit is homogenized by the fact that the coupling-out efficiency fromwaveguide plate 10 is greatest at that point where the light intensityis lowest. In the backlighting unit shown in FIG. 2A, however, thecoupling-out efficiency is determined not by the number of prisms but bythe dimensions of the prisms, the coupling-out efficiency rising as thecross-sectional area of the prisms increases. It must be considered inthis context, however, that the coupling-out efficiency reaches asaturation range when adjacent prisms intersect.

It is advantageous if the arrangement of prisms exhibits at least thesame symmetry elements as the lamp, the reflector, and the waveguideplate without prisms. It thereby is possible, with relatively littleoutlay, to achieve a high degree of homogeneity in the emitted light. Itis also possible, however, to select an asymmetrical prism arrangement,especially in terms of the longitudinal axis of the backlighting unit.In this case the intensity distribution of the light emitted from thebacklighting unit is also not completely homogeneous. In this fashion itis possible, and provision is made therefor, to deliberately generateinhomogeneities which replace the inhomogeneities generated by lightsource 12. It is also possible, however, and provision is made therefor,to compensate for the asymmetrical distribution of the prisms, byselecting the cross-sectional areas appropriately, in such a way thatthe backlighting unit once again emits homogeneous light.

It is furthermore possible, and provision is made therefor, to configureprisms 17 as prisms whose ridge lines are curved.

A further exemplary embodiment is shown in FIGS. 3A through 3C. FIG. 3Ashows a plan view of a backlighting unit. The backlighting unit onceagain has an approximately parallelepipedal waveguide plate 10 made of atransparent material, for example Plexiglas. As in FIG. 1A, waveguideplate 10 is surrounded on three of its end faces by a light source 12,behind which is located a reflector 13. In contrast to the previousFigures, light source 12 surrounds one long side and two short sides ofwaveguide plate 10. Underside 16 of waveguide plate 10, visible in FIG.3A, is equipped with prisms 17 whose ridge lines are V-shaped, themirror axes of the prisms being arranged congruently with thelongitudinal axis of light source 12 (i.e. parallel to the two arms ofthe U-shaped light source 12). The prisms have the same cross-sectionalshape and cross-sectional area throughout, both with respect to oneanother and also along their longitudinal axis (with the exception ofthe inflection region, in which the cross-sectional shape is morecomplex).

The spacing between the prisms is smaller in the region of waveguideplate 10 which is located farthest from light source 12.

This is also illustrated in the two cross-sections in FIGS. 3B and 3C.

FIG. 3B shows a section through the backlighting unit depicted in FIG.3A along section line HH′ which is also depicted in FIG. 3A, identicalcomponents having been given identical reference characters.

FIG. 3C shows a further section through the backlighting unit depictedin FIG. 3A, in this case along section line II′ which is also reproducedin FIG. 3A.

For a functional description of the backlighting unit depicted in FIG.3A, reference is made to the description of the backlighting unitdepicted in FIG. 1A. Once again, the light emitted from the backlightingunit is homogenized by the fact that the coupling-out efficiency fromwaveguide plate 10 is greatest at that point where the light intensityis lowest. As was already the case in the exemplary embodiment shown inFIG. 1A, the coupling-out efficiency is increased by decreasing thespacings between the prisms. In contrast to the exemplary embodimentshown in FIG. 1A, in this case the ridge lines are arrangedapproximately perpendicular to the gradient (along the so-calledequal-intensity lines) of the light intensity in waveguide plate 10, thecoupling-out coefficient being increased along the gradient byincreasing the prism density.

As a modification of the exemplary embodiment in FIG. 3A, it isconceivable, and provision is made therefor, to provide curved ridgelines for the prisms so that the prisms are guided more closely alongthe equal-intensity lines.

FIG. 4A shows a further exemplary embodiment of a backlighting unitaccording to the present invention, in a plan view of light exit surface14. The backlighting unit once again has an approximatelyparallelepipedal waveguide plate 10 which is made from transparentmaterial. Prisms 17 are not visible in this depiction, since they arelocated on the underside. Waveguide plate 10 is once again surrounded onthree sides by a U-shaped light source 12 which in turn is surrounded bya reflector 13. A reflective layer 15 is applied on the end face ofwaveguide plate 10 which is not equipped with a light source 12. Thebacklighting unit depicted in FIG. 4A is then depicted in four differentsectional views.

FIG. 4B shows a first section through the backlighting unit depicted inFIG. 4A, along section line EE′ which is located closest to reflectivelayer 15.

FIG. 4C shows a section through the backlighting unit depicted in FIG.4A along section line FF′, which runs parallel to section line EE′ andis arranged approximately in the center of waveguide plate 10.

FIG. 4D shows a section along section line GG′, which is also parallelto section line EE′ but runs close to the edge of waveguide 10 at thebase of the U-shaped light source 12.

Lastly, FIG. 4E depicts a section along section line HH′, section lineHH′ being coincident with the longitudinal axis of the backlightingunit.

All FIGS. 4A-4E show a waveguide plate 10 which is equipped at its endsurfaces with a backlighting unit 12 that is partially surrounded by areflector 13. The upward-facing light exit surface 14 is planar, whereasunderside 16 of waveguide 10 curves inward, resulting in a concave crosssection in FIGS. 4B through 4D. Arranged on underside 16 of waveguideplate 10 are prisms 17 which all have the same cross-sectional area andthe same cross-sectional shape, and extend parallel to section line EE′,i.e. perpendicular to the longitudinal axis of the illumination unit.

The curvature of waveguide plate 10 is selected so that the waveguideplate is thinner where the light intensity is somewhat lower. The lightis thus, so to speak, guided onto the prisms 17 in the regions of lesserlight intensity, which in turn results in increased coupling-outefficiency.

Modifications of the exemplary embodiment depicted in FIG. 4A arepossible, and provision is made therefor. For example, provision is madefor configuring waveguide plate 10 in such a way that the curvature isapplied on light exit surface 14, and the prisms on underside 16. Thismakes waveguide plate 10 easier to manufacture.

Provision is also made for prisms 17 on waveguide 10 to be rotatedapproximately 90 degrees, so that their ridge lines extend approximatelyperpendicular to the longitudinal axis of backlighting unit 12.

By mounting a mirror beneath the underside of waveguide plate 10, it ispossible to direct more light from the light source to the light exitsurface. FIG. 5 shows a mirror 18 that is provided for installationbeneath waveguide plate 10. In addition, a light source 12 is depictedwith dashed lines in FIG. 5 in order to elucidate the installationlocation of the mirror relative to the waveguide plate and the lightsource. The mirror is equipped at several spots with blackenings whichreduce the reflectance of the mirror at that spot. A decrease in thecoupling-out efficiency is thereby also achieved. If the blackenings areapplied at those points where the light intensity in waveguide plate 10is particularly high, the result of blackening 19 is a homogenization ofthe emitted light.

The term “blackening” as used above is intended to refer not only to theapplication of black color. “Blackening” in this context instead denotesa surface treatment in a manner suitable for reducing reflectance at thespots. This can be accomplished, for example, by etching the mirrorsurface, removing the metallization layer of the mirror, or applying anadditional layer which acts in either absorbing or scattering fashion oras an anti-reflection layer.

Further possible modifications of the invention result from combinationsof the various exemplary embodiments. For example, it is possible, andprovision is made therefor, also to use a double-paraboliccross-sectional shape for the prisms in the exemplary embodiments shownin FIGS. 2A through 4E, since that allows collimation of the coupled-outlight. A curved ridge line can also be used, allowing a more accuratelyadjustable homogenization of the light.

To improve the coupling-out efficiency, provision is made to equip theprisms additionally with a mirror layer. It is also possible, andprovision is made therefor, to select the roof angle of the prisms insuch a way that coupling out occurs below the particularly efficienttotal reflection. It is necessary in this case, however, for the lightemerging through the light exit surface to be aligned parallel to thesurface normal line. This can easily be accomplished by way of anadditional film which follows after the backlighting unit.

It is furthermore possible, and provision is made therefor, to use anL-shaped or O-shaped lamp, or also to apply prisms on the light exitsurface.

The prisms on the light exit surface can perform several functions. Onthe one hand, when the materials and the roof angle are selectedappropriately, they reduce reflection back into the waveguide plate atthe light exit surface, and thus enhance the coupling-out efficiency. Onthe other hand, light refraction at the side surfaces of the prismscauses the light to be directed in parallel fashion to a desired extent.

The prisms are preferably arranged on the light exit surface in such away that their ridge lines are approximately perpendicular to the ridgelines of the prisms arranged on the underside. It is also advantageous,and provision is made therefor, additionally to provide the prisms onthe light exit surface with a variable cross section or variabledensity, by analogy with the prisms arranged on the underside.

What is claimed is:
 1. A backlighting unit for a planar homogeneousillumination, comprising: a waveguide plate including at least twocovering surfaces and at least two narrow sides, the waveguide platebeing equipped with prisms which are located on at least one of the atleast two covering surfaces; and a light source situated on at least twoadjacent narrow sides, a light of the light source being capable ofbeing optically coupled into the waveguide plate, wherein first regionsof the waveguide plate have a first light intensity, a first number ofthe prisms per unit area and first dimensions for the prisms, whereinsecond regions of the waveguide plate have a second light intensity, asecond number of the prisms per unit area and second dimensions for theprisms, the first light intensity being lower than the second lightintensity, and wherein at least one of: i) the first number is greaterthan the second number, and ii) the first dimensions are larger than thesecond dimensions.
 2. The backlighting unit according to claim 1,wherein the light source includes a cold-cathode fluorescent lamp. 3.The backlighting unit according to claim 1, wherein the light source isU-shaped.
 4. The backlighting unit according to claim 1, wherein each ofthe prisms has a longitudinal axis.
 5. The backlighting unit accordingto claim 4, wherein each of the prisms has a constant cross-sectionalshape along the longitudinal axis.
 6. The backlighting unit according toclaim 5, wherein the prisms have an identical cross-sectional shape. 7.The backlighting unit according to claim 6, wherein the prisms have across-sectional shape of a triangle, each angle of the triangle having apredetermined value.
 8. The backlighting unit according to claim 7,wherein lateral lines of the triangle are curved.
 9. The backlightingunit according to claim 7, wherein lateral lines of the triangle areparabolically curved.
 10. The backlighting unit according to claim 4,wherein the longitudinal axes are locally arranged approximatelyparallel to a direction of a light intensity gradient.
 11. Thebacklighting unit according to claim 4, wherein the longitudinal axesare locally arranged approximately perpendicular to a direction of alight intensity gradient.
 12. The backlighting unit according to claim4, wherein the longitudinal axes are locally arranged approximatelyparallel, the first regions having a first depth of the prisms and afirst width of the prisms, the second regions having a second depth ofthe prisms and a second width of the prisms, at least one of the firstdepth and the first width being greater than at least one of the seconddepth and the second width.
 13. The backlighting unit according to claim12, wherein the longitudinal axes are oriented approximately parallel toa symmetry axis of the backlighting unit.
 14. The backlighting unitaccording to claim 13, wherein surfaces of the prisms are equipped witha reflective layer.
 15. The backlighting unit according to claim 1,wherein the first number of the prisms per unit area is greater than thesecond number of the prisms per unit area and the first dimensions forthe prisms are greater than the second dimensions for the prisms.
 16. Abacklighting unit for a planar homogeneous illumination, comprising: awaveguide plate including at least two covering surfaces and at leasttwo narrow sides, the waveguide plate being equipped with prisms whichare located on at least one of the at least two covering surfaces, theat least two narrow sides including at least two narrow sides that areadjacent to one another; and a light source situated on the at least twoadjacent narrow sides that are adjacent to one another, a light of thelight source being optically coupled into the waveguide plate, whereinfirst regions of the waveguide plate have a first light intensity, afirst number of the prisms per unit area and first cross-sectional areasfor the prisms, wherein second regions of the waveguide plate have asecond light intensity, a second number of the prisms per unit area andsecond cross-sectional areas for the prisms, the first light intensitybeing lower than the second light intensity, and wherein at least oneof: i) the first number is greater than the second number, and ii) thefirst cross-sectional areas are larger than the second cross-sectionalareas.